Transmission line monitor



TRANSMISSION LINE MONITOR Filed March 19, 1942 [ND/CA TOR AND ALARMC/RCU/T 2 51 INVENTOR T0 'EaRqELA/ssuumv INDICATOR BY ATTORNEY PatentedJan. 2, 1945 UNITED STATES "PATENT OFFICE George L. Usselman, RockyPoint, N. Y., assig'nor to Radio Corporation of America, a corporationof Delaware Application March 19, 1942, Serial No. 435,302 9 Claims.((11. 178-44) This invention relates to radio frequency transmissionlines and, more particularly, to protective devices or monitors for usewith such transmission lines.

Concentric transmission lines, such as are used with radio frequencytransmitters, increase rapidly in cost as sizes increase. Therefore, atransmission line of as small a size as possible is ordinarily used. Thesize of such line which is selected for a given transmitter output isordinarily governed by the maximum standing wave voltage which may occurduring short-circuit or open circuit of the line. Under certaincircumstances, such as when the line becomes temporarily improperlyterminated from some unforseen or uncontrollable cause, a standing wavemay occur which has an amplitude greater than that existing across theline during normal operation. Such action may occur, for example, if theload connected to said line changes its impedance due to iceaccumulation or accidents such as wires breaking, and so forth. In suchcases a standing wave voltage may result which exceeds the tolerablemaximum. Break down of, or other damage, to the line may then result. Toguard against this possibility it has been necessary to use transmissionlines of greater size than that dictated by the aforementioned factorsgoverning selection of a suitable line. The resultant increase in costmay be considerable. In the foregoing discussion the case of concentrictransmission lines has been particularly considered. However, thepresent invention may also be applied to open wire transmission lines toindicate the occurrence of improper matching of the load to the line orto prevent break down thereof.

An object of the present invention is to provide an improved protectivedevice for radio frequency transmission lines which willprotect suchlines from damage upon the occurrence of undesirable standing wavesthereon.

Another object of the present invention is to provide an improvedprotective device, as aforesaid, which will enable the use oftransmission lines of least expense.

A further object of the present invention is to provide a protectivedevice for transmission lines which upon the occurrence of standingwaves on said line will prevent their amplitude from exceeding apredetermined value.

Still another object of the present invention is to provide an improvedreflection indicating means for radio frequency transmission lines whichcauses no loss of power when normal conditions exist on the lines.

It is also an object of the present invention to provide a protectivedevice for a radio frequency transmission line, as'above set forth,which is highly efiicient and reliable in use, simple in constructionand inexpensive in cost.

In accordance with the principles of the pres-. ent invention aplurality of biased rectifiers are connected across the transmissionline at difierent distances from each other (for example, threerectifiers may be used spaced from each other at distances equal toone-fourth and one-eighth of the standing wave) and their outputsconnected to an alarm relay or to switching means for disabling thetransmitter connected to the line. These'rectifiers respond to increasesin line voltage above a predetermined maximum value at the points wherethey are respectively located. The value is determined by the biasapplied to the control grids of the rectifiers. Several rectifiersshould be used since in certain cases a single rectifier, or even tworectifiers, will not always detect an increase in the standing wave onthe line. The current in the common output circuit of the rectifiersincreases sharply a the standing wave voltage increases over thepotential for which the grid .bias constitutes cut-off value. This sharpincrease is caused to operate a relay in the output circuit to give thedesired indication. The

arrangement may be such thatlhaalarm llg y nag-qwll 'i nsmit thereby e li g flgim jlity of dama e to miss .thestan rthermore,

t e load or output circuit of the rectifiers may be so proportioned thatwhen the rectifiers draw current from the line the current drawn issufficient to prevent the potential acros the line from exceeding apredetermined value.

The present invention will be more fully understood by reference to thefollowing detailed description which is accompanied by a drawing whereinFigure 1 illustrates an embodiment of the present invention, and Figure2 illustrates a modification thereof.

Referring more particularly to the drawing wherein similar referencecharacters designate corresponding parts throughout there is shown inFigure 1 a transmission line including conductors Ill and I2. Thetransmission line, while it appears as a two-wire open line havingconductor Ill grounded, may of course equally well be a concentrictransmission line with the conductor I0 constituting the outer shell andconductor l2 the inner conductor. At one end of the transmission line isconnected a source of high frequency waves I4 and at the other end, aload l6.

As is common practice in the art, the impedance of load It is adjustedto equal the impedance of the transmission lien whereby standing wavesalong the line are substantially eliminated. A plurality of gridcontrolled rectifier tubes I! are connected at specified points alongthe transmission line. The distance between one pair of adjacentrectifier tubes is preferably one-quarter of the operating wavelength,while the other rectifier tube is spaced one-eighth of a wavelength awayfrom one of the other rectifier tubes. Thus a change in voltage mustalways occur across at least one of the rectifiers upon the occurrenceof standing waves on the line. In the arrangement shown each of therectifier tubes H has its anode I8 connected to conductor l2 of thetransmission line and its cathode l9 connected through condenser 22 tothe other side of the transmission line. Each of the cathodes I9 areconnected together and to a load circuit including resistor 23 andwinding 24 of a relay 25. Each of the grids 20 of rectifier tubes I! areconnected together and to a movable tap 26 on potentiometer 21.Potentiometer 27 is connected across a biasing battery 28. By means ofthis arrangement any predetermined bias may be placed on the grids 20with respect to cathodes 19. Each of the grids 20 of tubes I! areby-passed to a ground conductor it of the transmission line by means ofbypass condensers 30. A coil 32 having an electrical length equal to aquarter wavelength is connected across the transmission line at its endadjacent source I4 in order to complete the direct current circuit forthe rectifier tubes. The movable tap 26 on potentiometer 2'! is set tosupply a negative grid bias voltage such that substantially no platecurrent is drawn when no reflections are present on the radio frequencytransmission line. Now, when reflections do oc our on the transmissionlines for any reason and if they are large enough, the anode voltage onone or more of the rectifier tubes increases sufficiently to overcomethe fixed grid bias. Recti- ,fied current flows through the tube by wayof the resistor 23, the actuating winding 24 of relay 25 and half wavecoil 32, thus closing the contacts of the relay. Closure of the relaycontacts energizes a circuit to operate a warning lamp or alarm bell orother indicating devices. The resistance 23 is provided in order tolimit the space current through tubes l8 should the reflection voltagesbe exceedingly high. If the current carrying capacity of tubes I8 issufiicient the resistor 23 may be made of such value as to prevent thepeak voltages across the line from exceeding a predetermined minimumvalue. It may be seen that increasing the bias on the grid of tube I!will allow some reflection to take place on the line before the alarmoperates. This reflection may be made any desirable amount by the biasadjustment.

In Figure 2 is shown a modification of the embodiment shown in Figure 1wherein the transmission line is balanced with respect to ground, thatis, conductors 40 and 42 f the transmission line are each at a potentialdilferent from the ground potential. The source l4 and the load It areas before described. Pairs of grid controlled rectifier tubes 44 areprovided, each tube having its plate connected to one of the twotransmission line conductors and its grid connected to the movable tap26 of potentiometer 21 as described with reference to Figure 1. Sinceneither of the conductors of the transmission line are at groundpotential, a pair of quarter wave coils 46, 48, connected in seriesacross the line and with their center connections grounded, are providedinstead of the single quarter wave coil 32 of Figure l. The coils form areturn path for the rectified current flowing through rectifiers 44 fromeither side of the line to ground. The operation of the embodiment ofFigure 2 is substantially the same as described in detail with respectto Figure 1. 7

From the foregoing description it will be apparent to those skilled inthe art that I have provided a simple and effective way of protectingradio frequency transmission lines from becoming damaged as a result ofexcessive voltages resulting from the occurrence of standing waves onthe line.

While I have shown and particularly described several embodiments of myinvention, it is to be distinctly understood that my invention is notlimited thereto but that modifications within the scope of my inventionmay be made.

I claim:

1. A radio frequency system including a transmission line having a pairof conductors, a plurality of rectifiers having input circuits connectedat spaced points to a conductor of said line, said rectifiers having acommon output circuit including a control device operative in responseto a current flow through said circuit and means for biasing saidrectifiers inoperative in the presence of normal potentials on saidline, said rectifiers being so spaced along said line that upon theoccurrence of standing waves on said line at least one of them is at apotential loop.

2. A radio frequency system including a transmission line having a pairof conductors, a plurality of rectifiers having input circuits connectedat spaced points to a conductor of said line, said rectifiers having acommon output circuit including a control device operative in responseto a current flow through said circuit and means for biasing saidrectifiers inoperative in the presence of normal potentials on saidline, a pair of said rectifiers being spaced apart a distance equal toan eighth of the operating wavelength.

3. A radio frequency system including a transmission line having a pairof conductors, a plurality of rectifiers having input circuits connectedat spaced points to a conductor of said line, said rectifiers having acommon output circuit including a control device operative in responseto a current flow through said circuit and means for biasing saidrectifiers inoperative in the presence of normal potentials on saidline, said rectifiers being so spaced along said line that upon theoccurrence of standing waves on said line at least one of them is at aposition other than a potential node.

4. A radio frequency system including a transmission line having a pairof conductors, a plurality of rectifiers having input circuits connectedat spaced points across said line, said rectifiers having a commonoutput circuit including a control device operative in response to acurrent fiow through said circuit and means for biasing said rectifiersinoperative in the presence of normal potentials on said line, saidrectifiers being so spaced along said line that upon the occurrence ofstanding waves on said line at least one of them is at a potential loop.

5. A radio frequency system including a transmission line having a pairof conductors, a plurality of rectifiers having input circuits connectedat spaced points across said line, said rectifiers having a commonoutput circuit including a control device operative in response to acurrent; flow through said circuit and means for biasing said rectifiersinoperative in the presence of normal potentials on said line, saidrectifiers being so spaced along said line that upon the occurrence ofstanding waves on said line at, least one of them is at a position otherthan a potential node.

6. A radio frequency system including a transmission line having a pairof conductors, a plurality of grid controlled rectifiers having inputcircuits connected at spaced points to a conductor of said line, saidrectifiers having a common output circuit including a control deviceoperative in response to a current flow through said circuit and meansfor applying such bias to the grids of said rectifiers that saidrectifiers are inoperative in the presence of normal potentials on saidline,

said rectifiers being so spaced along said line that upon the occurrenceof standing waves on said line at least one of them is at a potentialloop.

7. A radio frequency system including a transmission line having a pairof conductors, a plurality of grid controlled rectifiers having inputcircuits connected at spaced points from a conductor of said line toground, said rectifiers having a common output circuit including acontrol device operative in response to a current flow through saidcircuit and means for applying such bias to the grids of said rectifiersthat said rectifiers are inoperative in the presence of normalpotentials on said line, a pair of said rectifiers being spaced apart adistance equal to a quarter of the operative wavelength and anotherrectifier being spaced from one: of said pair a distance equal to aneighth of the operating wavelength.

8. A radio frequency system including a transmission line having a pairof conductors, a plurality of grid controlled rectifiers having inputcircuits connected at spaced points to a conductor of said line, saidrectifiers having a common out- I put circuit including a control deviceoperative in response to a current fiow through said circuit and meansfor applying such bias to the grids of said rectifiers that saidrectifiers are inoperative in the presence of normal potentials on saidline,

, said rectifiers being so spaced along said line that upon theoccurrence of standing waves on said line at least one of them is at aposition other a potential node.

9. A radio frequency system including a transmission line having a pairof conductors, a plurality of grid controlled rectifiers having inputcircuits connected at spaced points to a conductor of said line, saidrectifiers having a common output circuit including a control device aoperative in response to a current flow through GEORGE L. USSELMAN.

